Voters in Jefferson County appeared to choose Mike Revis, a 27-year old Democrat, to fill a seat left vacant when the incumbent quit to run for county executive. With all 10 precincts within the district reporting, Revis led Republican David Linton by 108 votes, or about 3 percentage points.

If Revis's lead holds, it would mark a significant swing from 2016, when Trump won the district by a 61 percent to 33 percent margin.

Democrats will portray the unexpected win Tuesday as more evidence that the party's voters are fired up ahead of November's midterm elections, after a string of special election wins over the course of the last year.

Revis is the second Democrat this year to win a Republican-held seat in a state legislative race, after Democrat Patty Schachtner won a state Senate seat in Wisconsin.

Republicans easily won two other special elections to fill House vacancies on Tuesday, and the GOP still enjoys a supermajority in the state House.

One more district, a rural seat about halfway between St. Louis and the Arkansas border, remained too close to call as of 10 p.m. EST. In that race, Democrat Jim Scaggs led Republican Chris Dinkins by just 66 votes out of about 4,100 cast, with 10 precincts yet to report. President Trump won that district by an incredible 78 percent to 19 percent margin, making the narrowness of the race even more remarkable.

But the Democratic win is sure to startle Republicans who hope to hold on to control of key legislative seats in suburban and exurban districts around the country.